A role for vitamin D in the pathogenesis and treatment of Mycobacterium leprae (mLEP) has been presumed for many years. Pafients suffering from the progressive, bacilli-abundant lepromatous form (L-lep) ofthe disease are more likely to be vitamin D-deficient and benefit clinically from ultraviolet B (sunlight) irradiation or dietary supplementation vitamin D. Such patients are also at risk for developing dysregulated over- production of the active vitamin D metabolite 1,25-dihydroxyvitamin D (1,25D) from circulating 25- hydroxyvitamin D (25D) by disease-activated macrophages. On the other hand, intracellular 1,25D synthesis and action at the level of the vitamin D receptor (VDR) is crucial for mounting an antimicrobial response to mLEP. Therefore, the mechanism(s) that govern vitamin D metabolism and action in leprosy is a key component to the disease. Our recent studies In vitro clearly demonstrate the differential expression of the functional elements ofthe vitamin D system, including CYP27B1-hydroxylase and the VDR in Type I and II interferon-driven, T-lep and L-lep granulomas, respectively. Taken together, these clinical and laboratory findings lead us to theorize that mLEP Infection and the respective T-lep or L-lep downstream interferon- directed pathways, will differentially Impact the synthesis, metabolism and function of active vitamin D metabolites in the macrophage and other elicited, VDR-expressing inflammatory cells in the infectious microenvironment ofthe host with leprosy. To test this hypothesis, we will undertake three conceptually novel, mechanistic aims. First, the vitamin D system components (CYP2R1, vitamin D hydroxylase; CYP27B1; CYP24A1, 24-hydroxylase; and VDR) will be quantitatively mapped in T-lep and L-lep granulomas at the single cell level. Second, the orchestrated effects of T-lep or L-lep immune response secretomes, and associated downstream interferon responses, on the metabolism and immunoaction of vitamin D in human inflammatory cells will be characterized using innovative molecular tools developed in Projects 1 and 3. Third, using recently-conceived RNA sequencing technologies, the functional consequences ofthe human host vitamin D deficient state, and its rescue In vitro and in vivo, on immune responses to and killing of mLEP will be probed. When analyzed in concert with the experimental results of the other CORT projects, it is anticipated that this work will set the stage for the practice of manipulating human vitamin D balance in promotion ofthe innate and adaptive immune response in leprosy specifically and in granuloma-forming diseases in general.